U.S. Pat. No. 4,923,720 issued May 8, 1990, describes an invention for the liquid spray applications of coatings such as lacquers, enamels and varnishes using an environmentally safe, non-polluting diluent that can be used to thin highly viscous polymers and coating compositions to liquid spray application consistency. The '720 patent describes the utilization of supercritical fluids, such as supercritical carbon dioxide fluid as diluents in highly viscous organic solvent borne and/or highly viscous non-aqueous dispersion coating compositions to dilute these compositions to application viscosity required for liquid spray techniques, thereby substantially reducing the amount of volatile organic compounds (VOC) emissions from coating applications. To the extent a knowledge of the supercritical fluid phenomena is necessary for the understanding of this invention and the need that it fulfills, the disclosure of the '720 patent is incorporated herein by reference.
Apparatus for practicing the invention described in the '720 patent is described in U.S. patent application Ser. No. 413,517, filed Sep. 27, 1989. Apparatus and methods are disclosed for accurately and continuously providing a proportionated mixture comprised of non-compressible fluid and compressible fluid for spraying upon a substrate to be coated, relying particularly upon mass proportionation, to obtain the desired mixture of the compressible and non-compressible fluids.
As used in that application and as used herein the phrase "compressible fluid" is meant to include a material whose density is affected by a change in pressure to an extent of at least about 2 percent.
Specifically, the mass flow rate of the compressible fluid is continuously and instantaneously measured. Regardless of what that flow rate is and whether or not it is fluctuating as a result of, for example, being pumped by a reciprocating pump or regardless of the state of such compressible fluid, that mass flow rate information is fed to a signal processor on a continuous and instantaneous manner. Based on that received information, the signal processor in response to the amount of compressible fluid that has been measured, controls a metering device which controls the rate of flow of the non-compressible fluid. The non-compressible fluid is then metered in a precise predetermined proportion relative to the compressible fluid flow rate such that when the compressible and non-compressible fluids are subsequently mixed, they are present in the admixed coating formulation in the proper proportions.
The apparatus and methods disclosed in U.S. patent application Ser. No. 413,517, however, are particularly effective and specifically focused for producing the desired proportionated mixture of compressible and non-compressible fluids on a relatively large scale, continuous basis. The inventions disclosed in that Application are most suitable for substantially large industrial facilities wherein the substrate to be coated typically is transported on a conveyor system past one or more spray guns, which may be stationary or moving, to be sprayed by the apparatus disclosed therein. Such systems, and the like, may generally be used to coat automobile parts; electric motors; containers; pipe; coil steel, paper, fabric and other materials that are coated as they are rewound; plywood; porcelain enameling stove parts; adhesive on panels and honeycomb for laminating; sheet metal parts such as washers, dryers, refrigerators and the like; automotive bodies; furniture, case goods; and heavy machinery.
There are applications, however, that the continuous apparatus of the scale and sophistication envisaged in U.S. patent application Ser. No. 413,517 cannot meet in a practical and economical manner. Thus, for example, the automobile refinish industry, and small "end-use" shop and field spraying operations, and the like, where "economics-of-scale" dictate low cost equipment and a simple mode of operation, cannot effectively utilize the type of equipment disclosed in U.S. patent application Ser. No. 413,517. What was needed was a simple, semi-continuous method and apparatus, which was portable and small in scale.
Although smaller in size, such a system still must be able to feed, accurately proportion, pressurize, heat and mix a plurality of fluids, particularly one or more compressible fluids with one or more non-compressible fluids, and then be able to spray such mixed, heated and pressurized fluids through a spray gun. Most preferably, this relatively small-scale, semi-continuous unit should be able to accurately proportion, pressurize, heat and mix a coating material with a supercritical fluid, such as supercritical carbon dioxide, and spray such a mixture at supercritical conditions. Moreover, the apparatus should also be able to avoid settling of the contents of the mixed fluids such as when preparing, for example, a pigmented coating system; be easily cleaned when color changes are necessary; minimize the amount of solvent emissions to the environment; have a minimum of dead space; provide for circulating the coating fluid continuously through the spray system and gun to maintain precise temperature and pressure control; and have a minimum of moving parts requiring seals from which leaks may occur.
That need was met in U.S. patent application Ser. No. 544,777, filed Jun. 27, 1990, now U.S. Pat. No. 5,098,194 which describes apparatus which is more economical and practical for use in automobile refinishing and small "end-use" shop and field spraying operations and the like where "economics-of-scale" dictate low cost equipment and a simple mode of operation. The apparatus therein described is portable and small in scale; for example, about but not restricted to a fluid output of about 0.01 to 0.2 gallons per minute and a total capacity of about 0.1 to 1 gallon in the spraying system.
The invention described in U.S. patent application Ser. No. 544,777, now U.S. Pat. No. 5,098,194, involves introducing two or more fluids into two or more vessels capable of being pressurized. The fluids are then oscillated from one vessel to the other. Mixing of the fluids is accomplished by the oscillation itself. Pressurization of the vessels is accomplished by a hydraulic system.
Although the apparatus of U.S. patent application Ser. No. 544,777, now U.S. Pat. No. 5,098,194, satisfied many of the needs for smaller scale applications discussed above there still remains a need to simplify the apparatus. The above apparatus utilizes an hydraulic system to accomplish pressurization, mixing and circulation of the fluids. Additionally, the apparatus uses two high pressure accumulators and an oscillation loop to provide mixing. The use of a hydraulic system complicates the apparatus and the hydraulic fluid is a potential source of contamination of the coating material.
Accordingly, there is a need for a small portable batch-type apparatus that can achieve the results of the prior apparatus, but in a simpler and more economical manner.